Processes for the preparation of isocyanates

Abstract

Processes are described which comprise: (a) reacting chlorine with carbon monoxide to form phosgene; (b) reacting the phosgene with at least one organic amine to form at least one isocyanate and hydrogen chloride; (c) separating the hydrogen chloride; (d) oxidizing the hydrogen chloride with oxygen in a gas phase to form additional chlorine; and (e) recycling at least a portion of the additional chlorine to the reaction of the chlorine and the carbon monoxide; wherein the oxidation of the hydrogen chloride is initiated via a high energy source which may be selected from electron-exciting radiation, ionizing radiation, a gas discharge, a plasma, and combinations thereof.

Description

BACKGROUND OF THE INVENTION [0001] Chlorine is very often employed as an oxidizing agent in the production chain for the preparation of many organic compounds and for the preparation of raw materials for the production of polymers. Hydrogen chloride is often formed as a by-product in such reactions. For example, chlorine is employed in the preparation of isocyanates and hydrogen chloride is formed as a by-product. The hydrogen chloride can be utilized further, e.g., by marketing of an aqueous solution (hydrochloric acid) or by use in syntheses of other chemical products. [0002] However, the amounts of hydrogen chloride produced as a by-product cannot always be marketed or employed for other syntheses in their entirety. Furthermore, hydrogen chloride can normally be employed for synthetic purposes only if it has been appropriately purified beforehand. The marketing and sale of by-product hydrogen chloride is usually economical only if the hydrogen chloride or the hydrochloric acid do...

AI Technical Summary

Benefits of technology

[0007] One object of the present invention includes providing a process for the preparation of isocyanates which can include at least partial recycling of the hydrogen chloride obtained during the preparation of the isocyanates, and which can be operated easily and reliably. Additionally, such processes should preferably provide stable operation of the oxidation of hydrogen chloride without prior expensive purification of the hydrogen chloride.

[0008] It has now been surprisingly found that, in contrast to the known thermally-activated, heterogeneously-catalyzed process (Deacon process), by utilizing non-thermal excitation sources, the activation energy necessary for the reaction can also be supplied without a catalyst or supply of thermal energy, it being possible for the involved prepurification of the hydrogen chloride before the oxidation to be omitted and, where appropriate, it being possible to achieve a higher yield than according to the prior art.

Problems solved by technology

However, the amounts of hydrogen chloride produced as a by-product cannot always be marketed or employed for other syntheses in their entirety.

Disposal of hydrogen chloride, e.g., by neutralization with alkali, can be unattractive for economic and ecological reasons.

A disadvantage of the known heterogeneously catalysed oxidation of hydrogen chloride (Deacon process) is the incomplete conversion (up to 90%) at the reaction temperature required for the reaction with the known catalysts, which is conventionally in the range of between 250 and 450° C. The product mixture of the reaction is therefore always worked up in an involved manner.

A farther considerable disadvantage of the catalytic hydrogen chloride oxidation (Deacon process) is that the catalysts employed for the reaction are often exceptionally sensitive to impurities in the hydrogen chloride.

The recycling capacity rapidly drops drastically as a result of a loss in the activity of the catalyst.

This reduces overall the profitability of the catalytic oxidation process significantly.

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